1. Field of the Invention
The present invention relates to an improvement in an ultrasonic transducer, namely an ultrasonic probe to realize a high resolution ultrasonic diagnostic equipment by sharpening ultrasonic beam width in the direction of elevation orthogonally crossing the azimuth plane (i.e., the direction of Y axis).
2. Description of the Related Art
An ultrasonic transducer array, i.e. an ultrasonic probe arranging a plurality of rectangular transducer elements (hereinafter referred to as transducer elements) is widely used as a probe for electronically scanning an ultrasonic beam. In such an ultrasonic probe, a narrow beam has been required for near field to far field in order to realize such high resolution ultrasonic diagnostic equipment. Improvement of the resolution characteristic in the array direction (i.e. azimuth direction) has been conducted by electronic control of phase or amplitude of the transmitting or receiving wave of each transducer element, while that in the Y axis direction has been conducted by an acoustic lens. However, the beam width in the Y axis direction has a problem in that the beam becomes wide in fields other than the vicinity of the focal point of the acoustic lens.
Therefore, the following method has been employed in order to improve the beam characteristic in the Y axis direction from near field to far field.
FIG. 1(a) is a perspective view of an ordinary ultrasonic transducer array, i.e. an ultrasonic probe arranging a plurality of rectangular transducer elements 1. These rectangular elements are formed by dicing the piezo-electric ceramic plate having electrodes on its two surfaces, along the Y direction. The electrode on one of the surfaces is led out to the apparatus body by a flexible print card FPC 4 as a ground electrode, while the electrode on the other surface is led out as a signal electrode. The surface radiating the ultrasonic power (towards the upper side in FIG. 1(a) is generally the ground electrode; however, signal electrodes, which should not actually be seen, are drawn on the side of the radiation surface throughout the drawings for convenience of explanation.
FIG. 1(b) shows the signal electrode pattern, namely, the aperture shape of each transducer element 1 and its shading function which indicates the weight of radiation power. The weight is substantially proportional to electrode width in the X direction. Therefore, in the case of the rectangular electrode of FIG. 1(b) where the shading function is flat, no weighting is conducted. The azimuth plane is a plane in which ultrasonic beam scans in the axial direction (Z direction) perpendicular to the surface of transducer array, as shown in FIG. 1(a). An acoustic lens 3 is provided to narrow the ultrasonic beam width in the Y axis direction. The ultrasonic beam width, when the focal distance is 140 mm, is shown in FIG. 2, where beam widths of the beams radiated from a probe 20 mm wide in the Y direction are -10 dB and -20 dB lower than the center value as shown by curves (A) and (B), respectively. As is apparent from this figure, a narrow beam can be obtained in the vicinity of the focal distance 140 mm of the lens; however, the beam width becomes wider in the nearer or farther field than the focal distance of lens.
As a method of improving the ultrasonic beam characteristic, a probe which is structured so that the Y direction width of the transducer element, namely the aperture, is selected depending on the diagnostic distance, is shown in FIG. 3, where the signal electrodes of the transducer element are divided into A, B and A'. The central signal electrode B is selected for diagnosis of near field, i.e. at a distance shorter than the focal distance, and signal electrodes A, B and A' are used for diagnosis of far field, i.e. at a distance longer than the focal distance. This method accomplishes ultrasonic beam characteristics in which the -10 dB beam width (A) is improved around the focal distance; however, the -20 dB beam width (B) is not improved yet (see FIG. 4). FIG. 5 shows a third prior art arrangement such as disclosed in U.S. Pat. No. 4,425,525, in which the beam width is further narrowed by weighting the radiation power along the Y direction. In this case, the radiation power is weighted by varying the signal electrode width (diamond shape in FIG. 5) in the longitudinal direction (Y direction) of each transducer element, as shown in the shading function of FIG. 5. As a result, as shown in FIG. 6, the -20 dB beam width (B) before and after the focal point of the lens, is improved; however, the improvement of the -10 dB beam width (A) in the near field before the focal point is still insufficient.
FIG. 7 is a diagram for illustrating a fourth prior art method combining the method of FIG. 3 and the method of FIG. 5. As shown in FIG. 8, the -10 dB width (A) in the near field before the focal point is improved; however, there is a problem left unsolved in that the improvement of the -20 dB beam width (B) is still small, since the weighting is insufficient when only the signal electrode B is selected.